This application claims the priority of German patent document 198 19 428.5, filed Apr. 30, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an igniting element for an ignition circuit such as is provided in an air bag igniter.
In current air bag systems, the air bag igniters are ignited individually by a central control via a discrete line. For this purpose, an ignition capacitor, two ignition switches as well and a diagnostic logic per ignition circuit are included in the control unit. The principal disadvantages of this design: i) only approximately one third of the energy stored in the ignition capacitor is transferred to in the igniting element (approximately 6 .OMEGA. total resistance at 2 .OMEGA. ignition element resistance), ii) the energy converted in the ignition switches must be discharged to the substrate by good thermal conduction, iii) the surface of the ignition switches is determined by the switching current, the converted energy and the heat dissipation, and iv) the implementation of the circuit in silicon demands a double surface requirement for the ignition switches and the igniting element (German Patent Document DE 37 17 149 A1).
In each case, the diagnostic logic tests the function of the individual ignition switches, the total specific resistance of the ignition circuit and the connections of the line toward plus or ground.
Newer concepts of air bag ignition include a semiconductor ignition circuit which is coupled directly to the igniter, and which is remote-controlled from the central control unit by way of an ignition bus. This eliminates the specific resistance of the ignition circuit. Nevertheless, a ratio of approximately 1:1 still remains between the power dissipated at the ignition switches and the actual ignition output. The power dissipated at the ignition switches significantly influences the size of the ignition circuit constructed as an integrated semiconductor circuit and the size of the ignition capacitor.
It is an object of the invention to provide an ignition circuit which reduces the amount of power dissipation, the space requirement.
Another object of the invention is to reduce the costs of an air bag ignition circuit.
These and other objects and advantages are achieved by the ignition element according to the invention, which uses the "dissipation power" of the ignition switches to support the ignition. That is, the ignition switches themselves represent the igniting element, and the igniting element as an additional component is eliminated. The advantages of the solution according to the invention are that the dissipation power is equal to zero because the ignition switches themselves represent the igniting element. When implemented in silicon, only a single surface is required, and this surface requirement can be further reduced by the use of "high-loss" ignition switches. The surface of the ignition switches depends only on the pyrotechnics to be applied.
The required ignition energy can advantageously be reduced by thermal insulation of the ignition switches with respect to the substrate. Moreover, diagnosis can take place as in the previous solution, by testing the functioning of each of the two ignition switches. However, the need to test the specific resistance is eliminated. As indicated below, as an alternative, a function test can be carried out by means of a diode.
A further improvement of the system can be achieved by using only one ignition switch, and/or by performing the diagnosis by means of a separate sensor element (diode) via dynamic temperature measurement, so long as the test signal is below the ignition threshold. An additional advantage of this arrangement is that, for implementation in silicon, only a single, small amount of space is required. In this case, the diagnosis advantageously takes place by dynamic temperature measurement when the test signal is generated by the diagnostic logic, thereby permitting an actual function test at temperatures below the ignition threshold.
If, for reasons of safety, redundant switching is required, using two separate switches so that a single fault does not trigger the igniter, a second ignition switch may be connected on the input side. Another advantage of this arrangement is technological redundancy can be achieved by using different technologies for the two ignition switches.
For the silicon implementation of the igniting element according to the invention, a "silicon hotplate" structure is advantageously used to generate the required thermal ignition energy.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.